This application claims the benefit of and priority to Japanese Patent Application P2001-124519, filed on Apr. 23, 2001, the entire contents of which is incorporated hereby by reference.
The present invention relates to antiskid brake control systems for motor vehicles for preventing wheel lock by increasing or decreasing the fluid pressures within wheel cylinders of vehicle wheels during braking, and more particularly, to technique of controlling the empty capacity of a reservoir for storing working fluid discharged from the wheel cylinders during pressure reducing control of the brake fluid pressure.
In typical technique of controlling the empty capacity of the reservoir in the antiskid brake control systems, drive-current control of an electric motor for driving a hydraulic pump for circulating working fluid stored in the reservoir to a master cylinder is carried out with a pulse width modulation (PWM) control output to make an electric motor output or number of revolutions controllable. And the amount of working fluid in the reservoir is estimated, wherein when the estimated amount becomes larger, the motor output is increased, whereas when it becomes smaller, the motor output is decreased, maintaining the empty capacity of the reservoir at a given value or more, thus allowing achievement of sufficient pressure reduction.
However, typical technique of controlling the empty capacity of the reservoir needs a large electric motor with controllable current as described above, leading to difficult reduction in size of the system with cost increase thereof. Further, the large electric motor requires heavy current to increase a motor output, resulting in feel degradation in terms of acoustic vibration performance. Furthermore, achievement of PWM control needs countermeasures against radio noise, resulting in complicated circuit configuration and enlarged size of a control unit itself due to arrangement of a radiating plate for eliminating heat generated in transistors.
It is, therefore, an object of the present invention to provide antiskid brake control systems which allow control of the empty capacity of the reservoir with reduction in manufacturing cost and size of the system, and prevention of degradation in acoustic vibration performance without generating radio noise, nor enlarging the control unit.
The present invention provides generally a brake control system for a motor vehicle, which comprises: a sensor which senses a speed of a corresponding one of wheels; a hydraulic unit which allows increase and reduction in a fluid pressure within a wheel cylinder provided to the wheel, the hydraulic unit comprising a reservoir for storing a fluid discharge from the wheel cylinder during pressure reduction; and an electronic control unit (ECU) connected to the sensor and the hydraulic unit, the ECU being so constructed as to: calculate a pseudo vehicle-body speed in accordance with the sensed wheel speed; calculate a target speed of the wheel in view of a predetermined slip ratio obtained in accordance with the calculated pseudo vehicle-body speed; carry out a pressure reducing control by reducing the fluid pressure within the hydraulic unit when the sensed wheel speed is smaller than the calculated target wheel speed; estimate an amount of the fluid stored in the reservoir; and correct the target wheel speed to lessen the slip ratio when the estimated fluid amount is greater than a predetermined value.